“…we conclude that the target to limit global warming to 2°C, confirmed by the 2009 Copenhagen Accord of the 15th Conference of the Parties of the United Nations Framework Convention on Climate Change, would lead to disastrous consequences. For example, Earth’s history shows that 2°C global warming is likely to result in eventual sea level rise of the order of six meters (20 feet). Moreover, we note that such a warming level would induce ‘slow amplifying feedbacks’”. —James Hansen and Pushker Kharechaet

This post is an abridged version of a scientific paper, focusing on the summary conclusions presented in the first half. To read the full 6-page report, click on the following linked title. The abridged version below includes added subheadings.

This paper, by an international team of scientists, is being published today (3/12/2013) in the open access journal PLOS ONE, where it is freely available. The paper points out the clear and present danger that today’s children may be handed a deteriorating climate with consequences out of their control. However, despite the fact that governments today seem to allow and encourage extraction of almost every fossil fuel that can be found, we suggest that “there is still opportunity for humanity to exercise free will.”

Here we (Hansen and Kharecha) first summarize some of the main conclusions reached in the paper. Then in a following discussion we provide our opinion concerning more detailed policy implications

We conclude that the widely accepted target of limiting human-made global climate warming to 2 degrees Celsius (3.6 degrees Fahrenheit) above the preindustrial level is too high and would subject young people, future generations and nature to irreparable harm.

Carbon dioxide (CO2) emissions from fossil fuel use must be reduced rapidly to avoid irreversible consequences such as sea level rise large enough to inundate most coastal cities and extermination of many of today’s species. Unabated global warming would also worsen climate extremes.

Impacts of higher global warming

In association with summer high pressure systems, warming causes stronger summer heat waves, more intense droughts, and wildfires that burn hotter. Yet because warming causes the atmosphere to hold more water vapor, which is the fuel that drives thunderstorms, tornadoes and tropical storms, it also leads to the possibility of stronger storms as well as heavier rainfall and floods.

There’s evidence that climate extremes are already increasing in response to warming

Observational data reveal that some climate extremes are already increasing in response to warming of several tenths of a degree in recent decades; these extremes would likely be much enhanced with warming of 2°C or more. We use evidence from Earth’s climate history and measurements of Earth’s present energy imbalance as our principal tools for inferring climate sensitivity and the safe level of global warming. The inferred warming limit leads to a limit on cumulative fossil fuel emissions.

It is assessed that humanity must aim to keep global temperature close to the range occurring in the past 10,000 years, the Holocene epoch, a time of relatively stable climate and stable sea level during which civilization developed. The world cooled slowly over the last half of the Holocene, but warming of 0.8°C (1.4°F) in the past 100 years has brought global temperature back near the Holocene maximum.

Fossil fuel sources must be strictly limited — immediately; additional carbon emissions will continue warming the climate system for 100,000 years; therefore, the more emissions, the warmer the planet, the greater the devastation

We note that policies should emphasize fossil fuel carbon, not mixing in carbon from forest changes as if it were equivalent. Most of the carbon from fossil fuel burning will stay in the climate system for of order 100,000 years. Of course carbon dioxide from deforestation also causes warming and policies must address that carbon source, but good land use policies could restore most of that carbon to the biosphere on a time scale of decades to centuries. However, maximum biospheric restoration is likely to be only comparable to the past deforestation source, so fossil fuel sources must be strictly limited.

CONCLUSION: Human-made warming could be stabilized at 1C but only if…

We conclude that human-made warming could be held to about 1°C (1.8°F) if cumulative industrial-era fossil fuel emissions are limited to 500 GtC (gigatons of carbon, where a gigaton is one billion metric tons) and if policies are pursued to restore 100 GtC into the biosphere, including the soil. This scenario leads to reduction of atmospheric CO2 to 350 ppm by 2100, as needed to restore Earth’s energy balance and approximately stabilize climate.

Limiting global warming to 2C “would lead to disastrous consequences

In contrast, we conclude that the target to limit global warming to 2°C, confirmed by the 2009 Copenhagen Accord of the 15th Conference of the Parties of the United Nations Framework Convention on Climate Change, would lead to disastrous consequences. For example, Earth’s history shows that 2°C global warming is likely to result in eventual sea level rise of the order of six meters (20 feet). Moreover, we note that such a warming level would induce “slow amplifying feedbacks”.

Amplifying feedback loops would be substantial if warming reaches 2C or more

These amplifying feedbacks include a reduction of ice sheet area, vegetation changes including growth of forests in high latitudes of Asia and North America that are now sparsely vegetated, and an increase of atmospheric gases such as nitrous oxide and methane. These slow feedbacks are small if climate stays within the Holocene range, but substantial if warming reaches 2°C or more.

If emissions continue to grow until 2020, reductions would have to be 15% per year to stay within a safe limit

Cumulative fossil fuel emissions through 2012 are 370 GtC and increasing almost 10 GtC per year. The current emission rate would need to decrease 6% per year to limit emissions to 500 GtC. If reductions had begun in 1995, the required reduction rate would have been 2.1% per year, or 3.5% per year if reductions had begun in 2005. If emissions continue to grow until 2020, reductions must be 15% per year to stay within the 500 GtC limit, which emphasizes the urgency of initiating emission reductions.

If we allow warming to reach 2C or higher, the situation would then be out of control because it would take many centuries for the ocean to cool down

The huge fossil fuel energy infrastructure now in place makes it practically certain that the 500 GtC limit will be exceeded. However, the need to come as close as possible to that target is made clear by the specter of likely climate impacts from 2°C warming. Although it is difficult to predict the timing of consequences such as large sea level rise, its eventual occurrence likely will be locked in if we allow warming to reach a level as high as 2°C. The situation would then be out of humanity’s control, because, even if the atmospheric CO2 amount declines, it would take many centuries for the ocean to cool down.

Extracting CO2 from the air, as some have suggested, is likely impractical

We draw attention to the difficulty, and possible impracticality, of extracting much CO2 from the air, once it becomes clear that an acceptable level of CO2 has been overshot. Specifically, we note that the American Physical Society estimates a cost for air capture of 1 GtC with current technology as about $2 trillion, thus about $200 trillion to remove 100 GtC. Improved technologies might reduce this cost, but fundamental energy considerations imply that extraction will be very costly and very unlikely to be deployed at a sufficient scale in the required time frame. At most CO2 extraction might help alleviate a modest overshoot of the safe CO2 level at a high cost to future generations.

A carbon fee or tax is an essential but not sufficient policy option for rapidly reducing emissions

The research team, which includes three economists, covers a broad range of fields and does not shy away from “connecting the dots” all the way to policy implications. It concludes that the essential underlying policy, albeit not sufficient, is a rising price on carbon emissions that allows the costs of pollution and climate change to be internalized within the economics of energy use. We note that a rising carbon fee collected from fossil fuel companies would improve economic efficiency, as it allows energy efficiency and alternative low-carbon and no-carbon energies to compete on equal footing. The resulting energy transformations would generate many jobs, especially benefitting nations still in economic recession.

An advantage of a carbon fee or tax is the relative ease with which it can be made global. An agreement among even a few of the largest economies (United States, China, European Union, Japan) could spur near-global agreement. Countries agreeing to have a rising carbon fee would likely place border duties on products from countries without a carbon fee, thus providing strong incentive for other countries to join.

Government should also support alternative carbon-free energy sources including advanced generation nuclear power

Governments should also support technology research, development and demonstration of carbon-free energy including advanced generation nuclear power as well as renewable energy, especially in view of the urgency with which emissions from coal and unconventional fossil fuels must be eliminated. (Unconventional fossil fuels include tar sands, shale-derived oil and gas, and methane hydrates.)

[Here ends, on page 3 of 6, the Summary]

[Following is the paper’s concluding paragraph from page 6 of 6 of the PDF document]

Our paper was initiated to provide the scientific basis for legal actions against national and state governments for not doing their job of protecting the rights of young people and future generations. A lower court ruling in the case against the U.S. federal government, suggesting that the “trust” doctrine does not give the court a constitutional basis for ordering actions on the executive branch, is now being appealed to a higher court. The appeal places greater emphasis on “equal protection of the laws” and “due process”, which the U.S. Constitution guarantees to all people. Amici briefs have been filed with the court concerning both the scientific and legal aspects.

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